The present invention relates to a vapor deposition method for the GaAs thin film by which it has been made possible particularly to manufacture the wafer with a small dispersion in the distribution of the carrier density of the film of deposited n-type crystals.
In general, for the vapor deposition for the GaAs thin film, a vapor deposition apparatus shown in FIG. 4 is used, wherein a high-frequency induction heating coil (5) is provided around the outer circumference of a reaction tube (1) with an introductory port (2) of source gases at the upper end and an exhaust port (3) of gases at the lower end through a cooling jacket (4) and a carbon susceptor (6) in the shape of truncated hexagonal pyramid is arranged in the reaction tube (1). With this apparatus GaAs substrates (7) are fitted onto the pyramid faces of the susceptor (6) and, allowing the susceptor to rotate in the direction of arrow mark, source gases are introduced into the reaction tube (1) from the introductory port (2) and allowed to eject from the exhaust port (3) at the lower end.
In this way, the substrates (7) are heated to a predetermined temperature by the heating coil (5) and, through the thermal decomposition of stock gases near the surface of the substrates (7), the crystals of GaAs are allowed to deposit onto the substrates (7).
As the source gases, organic gallium, for example, trimethyl gallium [Ga(CH.sub.3).sub.3 ] and arsine gas (AsH.sub.3) are used. The deposition conditions to allow the high resistance GaAs layer to deposit are generally that the supplying ratio (V/III) of the source gases of Ga(CH.sub.3).sub.3 and AsH.sub.3 lies between 10 and 20 and the temperature of the substrate is near 650.degree. C. Moreover, for the formation of n-type film (a carrier density of not less than 1.times.10.sup.16 cm.sup.-3), gas having an impurity possible to become the source for the supply of electrons as an ingredient element is to be added to both stock gases. As the impurity, sulfur is used most frequently and, as the gas containing this, hydrogen sulfide gas (H.sub.2 S) is used. By varying the flow rate of this gas, the concentration of electrons, that is, the carrier density of n-type crystals is controlled.
For example, for the deposition of epitaxial wafer used for field effect transistor, high resistance crystal film (hereinafter abbreviated as buffer layer) is allowed to deposit onto the GaAs substrate in a thickness of 2 to 3 .mu.m and n-type crystal film (hereinafter abbreviated as doping layer) is allowed to deposit thereon in a thickness of about 0.5 .mu.m. The buffer layer is made from non-dope crystals added no impurity intentionally and is allowed to deposit making the supplying ratio (V/III) 10 to 20 since the carrier density becomes lowest and the resistivity becomes lowest within a range of the supplying ratio (V/III) of AsH.sub.3 and Ga(CH.sub.3).sub.3 of 10 to 20. In succession H.sub.2 S is added to these gases and the doping layer is allowed to deposit at the same deposition temperature as that in case of the buffer layer.
Although the carrier density of the depositing doping layer can be controlled by the flow rate of H.sub.2 S to be added to source gases as described above, it depends also on other deposition conditions. Namely, the carrier density of the doping layer varys also with the supplying ratio (VIII) of the source gases of AsH.sub.3 and Ga(CH.sub.3).sub.3 and further with the flow rate of Ga(CH.sub.3).sub.3 or the temperature of the substrate. As a result, in the crystal deposition of the wafer with a large area, there has been a problem that the dispersion is caused in the distribution of the carrier density of the face of wafer due to the temperature distribution on susceptor, the difference in the decomposition ratio of AsH.sub.3 resulting from the location on susceptor, or the like.